The present invention relates to an electric accumulator having a positive nickel-hydroxide electrode, a negative electrode comprised of a hydrogen storage alloy, an interposed separator and an alkaline electrolyte.
It is known that by forming metal hydrides, numerous metals and alloys are able to absorb and store significant amounts of hydrogen. This ability is the basis for an energy storage system which, in addition to the conventional Ni/Cd accumulator or the lead-acid battery, has recently met with a great deal of interest; namely, the Ni/metal hydride accumulator.
In such systems, positive and negative electrodes are arranged in an alkaline electrolyte, and separated from one another, as is usual in other systems. However, in this arrangement, the positive electrode in principle corresponds to the positive nickel hydroxide electrode of a Ni/Cd accumulator.
According to present standards, for example, in accordance with U.S. Pat. No. 4,935,318, the active mass of the electrode contains, in addition to nickel hydroxide as the main component, a conducting medium in the form of nickel metal powder, cobalt metal powder, certain foreign hydroxides (especially cobalt hydroxide), and a binder. These constituents are mixed in a dry state. The dry mixture is then mixed with water, to form a paste, and spread into a highly porous three-dimensional nickel matrix.
When an electric current is applied to the negative electrode of a Ni/metal hydride secondary cell (Ni/MH), the active material (i.e., the metal or the alloy M capable of absorbing hydrogen) is charged by the absorption of hydrogen: EQU M+xH.sub.2 O+e.sup.- .fwdarw.MH.sub.x +xOH.sup.- ( 1)
During discharging, the stored hydrogen is released so that an electric current is generated: EQU MH.sub.x +xOH.sup.- .fwdarw.M+xH.sub.2 O+e.sup.- ( 2)
Both reactions are reversible.
A similar situation applies to the reactions taking place at the positive nickel hydroxide electrode. EQU Charging: Ni(OH).sub.2 +OH.sup.- .fwdarw.NiOOH+H.sub.2 O+e.sup.-( 3) EQU Discharging: NiOOH+H.sub.2 O+e.sup.- .fwdarw.Ni(OH).sub.2 +OH.sup.-( 4)
Ni/metal hydride batteries, even today, offer distinct advantages compared to conventional secondary batteries. This is because an environmentally friendly energy source, hydrogen, is used in conjunction with the well-established, in principle identical positive electrode of a Ni/Cd accumulator.
Ni/metal hydride batteries are gas-tight and maintenance-free. Due to rather advanced electrode fabrication technologies, such batteries have already attained energy densities of 50 Wh/kg. Here, the quality of the positive electrode plays a particularly important part, since it limits the capacity.
Prior to the introduction of rapidly chargeable Ni/Cd button cells, the conducting medium used in nickel hydroxide electrodes (instead of the nickel powder which is customary today) was graphite. The positive electrode mainly consisted of mercury (II) oxide. German Patent No. 1,771,420 likewise proposes an addition of graphite to metal oxide/hydrogen cells to improve their conductivity.
The addition of cobalt and cobalt oxides (owing to their conductivity), which only later became customary (in addition to nickel metal powder as the conducting material), promotes mass utilization of the nickel hydroxide electrode. Such additions also serve to set a discharge reserve on the negative electrode since, during the first charging of the cell, prior to oxidation of the Ni(OH).sub.2, stable conductive cobalt oxides (CoOOH) are formed having an oxidation potential lower than that of NiOOH. As such, if the end-of-discharge voltage does not drop significantly below 1 V and the total charge supplied (once) to form the cobalt oxides is retained by the negative electrode as an excess capacity (discharge reserve) with respect to the positive electrode, such additions will no longer take part in subsequent reactions. However, under extraordinary operating conditions (e.g. polarity reversal, exhaustive discharge), the conductive matrix may be reductively destroyed.
Most commercial Ni/metal hydride accumulators, whether they are round cells, prismatic cells or button cells, present a serious problem in that they generally cannot withstand the high temperature short circuit (HTSC) test which is conventionally performed by battery customers in the industry. In this test, the cells in the discharged state are loaded with a 2 .OMEGA. resistor and stored for 3 days at 65.degree. C. Then, a capacity test is carried out during a few charging/discharging cycles at room temperature. The test simulates long-term short-circuit behavior in electronic appliances. The high temperature shortens the duration of the test. In so doing, massive irreversible losses in capacity are found. Such losses are due to reductive destruction of the CoOOH conductive matrix as a result of the negative potential of the metal hydride electrode being impressed on the positive electrode during the short circuit.